1. Field of the Invention
The present invention relates to a personal display device, and more particularly, to a head mounted display (HMD) device worn on the head of an observer, which enables a three-dimensional image to be displayed.
2. Discussion of the Related Art
Recently, a demand for personal display devices has been increased with a tendency to a small-sized display device. For this reason, a display device has been developed, in which an image emitted from the small-sized display device is enlarged by optics, so that a virtual image is displayed. At this time, an observer can feel himself watching the image on a large-sized screen at a predetermined distance.
The aforementioned personal display device is called as a head mounted display (HMD) device since the display device is generally worn on the head of the observer in the same manner of wearing eyeglasses.
FIG. 1 is a view showing a related art head mounted display device using a spherical lens.
Referring to FIG. 1, the related art head mounted display device includes a liquid crystal panel (LCD panel) 11, a backlight 12, a half mirror 13 and a spherical mirror 14.
At this time, the liquid crystal panel 11 displays an image restored from an image record medium (not shown). Also, the backlight 12 is formed at the rear of the liquid crystal panel 11, and emits light toward the liquid crystal panel 11. Then, the half mirror 13 changes a light path including the image when the light pass through the liquid crystal panel 11, and the reflective spherical mirror 14 changes the light reflected from the half mirror 13 to the parallel light, so that the observer can see the image.
FIG. 1 shows only one set for directing the image to one eye of the observer. That is, a pair of sets is required to direct the image to both eyes of the observer.
An operation of the head mounted display device will be explained in brief.
First, the light emitted from the backlight 12 is directed to the liquid crystal panel 11. At this time, the light includes image information during passing through the liquid crystal panel 11. Then, the light including the image information is firstly reflected on the half mirror 13 to the reflective spherical mirror 14. Subsequently, the light incident on the reflective spherical mirror 14 is secondly reflected to the half mirror 13, and the light passing through the half mirror 13 is received on the eye of the observer, thereby generating the virtual image on the eye of the observer.
FIG. 2 is a view showing a related art head mounted display device using right-angled bar prism optics (RBPO).
Referring to FIG. 2, the related art head mounted display device includes a liquid crystal panel 21 and right-angled bar prism optics (RBPO) 23. Also, the RBPO 23 includes a bar type prism 23a and an eyepiece lens 23b. 
At this time, a tilted surface 23c is formed at the bottom of the prism 23a at an angle of 45 degree, and a coating film is formed on the tilted surface 23c to totally reflect the light. Then, the eyepiece lens 23b is formed to be opposite to the tilted surface 23c. 
Accordingly, the image from the liquid crystal panel 21 is totally reflected on the tilted surface 23c of the RBPO 23, and then is directed to the eye of the observer, thereby generating the virtual image 1 in front of the observer through the eyepiece lens 23b. 
However, the related art head mounted display device has the following problems.
In the head mounted display device shown in FIG. 1, the observer's field of vision is restricted by the reflective spherical mirror 14, so that the observer feels himself watching the virtual image 1 floating in a dark cave. Accordingly, the observer complained that the virtual image was displayed in a smaller size than a designed size.
Also, in the head mounted device shown in FIG. 1, the half mirror 13 is a semi-transmissive mirror. That is, the half mirror 13 has the problem in that the light emitted from the liquid crystal panel 11 is used about ¾ or less. The light emitted from the liquid crystal panel 11 is firstly lost in the half mirror 133 at a percentage of 50, and the light emitted from the spherical mirror is secondly lost during passing through the half mirror 13 at a percentage of 50. Accordingly, the light incident on the eye of the observer is about 25% or less.
In the head mounted display device shown in FIG. 2, an unusual reflection is generated in the RBPO 23, thereby generating a second virtual image 2. That is, some of the light emitted from the liquid crystal panel 11 is not incident on the tilted surface 23c, but is incident on a side of the RBPO 23. At this time, the light incident on the side of the RBPO 23 is unusually reflected to the eye of the observer, so that the observer is confused due to the unnecessary second virtual image 2.
In the head mounted display device shown in FIG. 1 and FIG. 2, the centroid of the device is distant from the face of the observer, so that the device easily slides down, thereby reducing a wearing comfort.
In a case of the head mounted display device shown in FIG. 1, there is a limitation in decreasing a width (d1) of the optics due to structure of the half mirror 13 and the spherical mirror 14.
Also, in a case of the head mounted display device shown in FIG. 2, there is a limitation in decreasing a width (d2) of the RBPO 23 since the RBPO 23 has to have a predetermined thickness for directing the light emitted from the display panel 21 to the tilted surface 23c. Accordingly, the centroid of the device is distant from the face of the observer, so that the device slides down on the face of the observer.